1. Field of the Invention
The invention relates to crosslinkable compositions based on silane-terminated polyurethanes, to processes for preparing them, and to their use as adhesives and sealants, especially as adhesives possessing high tensile shear strength.
2. Description of the Related Art
In adhesives applications which call for high tensile shear strengths from the cured adhesives, isocyanate-crosslinking PU adhesives are typically employed. These adhesives customarily comprise isocyanate-functional polyurethane polymers based on aromatic polyisocyanates. Such systems cure through a reaction of the isocyanate groups with (atmospheric) moisture.
Since PU adhesives cure via a chemical crosslinking reaction and at the same time are also able to attach via chemical bonds to numerous substrates (e.g., wood, metals, ceramic substrates, glass, etc.), they usually exhibit very good mechanical properties and are also relatively resistant in the face of external (weathering) effects such as moisture or direct water contact.
Isocyanate-crosslinking adhesives do, however, also possess a number of disadvantages inherent to these systems, some of these disadvantages being substantial. For example, one-component PU adhesive systems display cure rates that are generally no more than moderate. The isocyanate crosslinking can indeed in principle be accelerated sharply by appropriate catalysis. However, since such catalysis is able in principle to catalyze unwanted side-reactions of the isocyanate groups as well (e.g., formation of allophanates, uretdiones, isocyanurates, etc.), the corresponding systems then lack a sufficient shelflife.
Another disadvantage of the majority of isocyanate-crosslinking adhesives is the health-related classification, which ranges from sensitizing to toxic. A critical factor here, in particular, is the amount of monomeric isocyanates which remain in the uncured adhesive, and which are removable only with difficulty. This presents problems for the end user, i.e., the craftworker or do-it-yourself user, who comes into contact not only with the fully cured and hence isocyanate-free and entirely unobjectionable product, but also with the as yet uncured and hence still isocyanate-containing adhesive and/or with the monomeric isocyanates present in this adhesive. For the unpracticed home improver there is a particular risk here that the products may not be used expertly and/or properly. Additional hazards arise here from incorrect storage, such as storage within the reach of children, for example. With the professional craftworker, on the other hand, largely proper use and storage can be assumed. Here, however, the problem usually exists that the professional user is required very regularly indeed—possibly even a number of times a day—to work with the isocyanate-containing material, something which is potentially critical in view in particular of the aforementioned sensitizing and also possibly carcinogenic effects of isocyanates.
Somewhat more favorable in this respect are isocyanate-crosslinking adhesives which contain only very low levels of volatile isocyanates and which are therefore at least free from labeling requirements. These adhesives, however, are mostly based on aliphatic isocyanates, which in turn are less reactive. For applications where rapid setting of the adhesive is a factor, therefore, these adhesives are even more unfavorable than conventional PU adhesives.
An alternative curing technology which is finding application increasingly in the adhesives sector is that of silane crosslinking, where alkoxysilane-functional prepolymers, on contact with atmospheric moisture, initially undergo hydrolysis and then cure through a condensation reaction. The corresponding silane-functional—usually silane-terminated—prepolymers are entirely unobjectionable from the toxicological standpoint, in general.
Polymer systems which possess reactive alkoxysilyl groups have been known for a long time. On contact with water or atmospheric moisture, these alkoxysilane-terminated polymers are capable even at room temperature of undergoing condensation with one another, with elimination of the alkoxy groups. Thus adhesives based on alkoxysilane-crosslinking polymers, in the fully cured state, exhibit not only good properties of adhesion to a number of substrates, but also very good mechanical properties, since on the one hand they exhibit a certain tensile strength and on the other hand they may be highly elastic. The materials in question, accordingly, are suitable especially for sealants and also for elastic adhesives with moderate tensile shear strength.
Preference here is given in numerous applications to one-component systems (1K systems), which cure on contact with atmospheric moisture. The crucial advantages of one-component systems include in particular their very great ease of application, since in this case there is no need for the user to mix a variety of adhesive components. In addition to the time/work saving and the reliable avoidance of possible metering errors, there is also no need with one-component systems to process the adhesive/sealant within a usually decidedly narrow time window, as is the case with multicomponent systems after mixing of the two components has taken place.
Corresponding alkoxysilane-terminated prepolymers have been prior art for some time and are available commercially, for example, under the trade names GENIOSL STP-E (from Wacker-Chemie AG), MS-Polymer (from Kaneka), DESMOSEAL (from Bayer AG), or SPUR (from Momentive).
A disadvantage of the majority of common silane-crosslinking systems, however, is the fact that while moderate tensile shear strengths are achievable, situated typically within an order of magnitude of 1-4 MPa, tensile shear strengths >5 MPa are unachievable or are achievable at best only in very unusual formulations. These unusual formulations, however, have other restrictions in turn, an example being a black coloration owing to the use of carbon black filler, as described in WO 02/090411, for example. Moreover, even with the formulations described in WO 02/090411, it is impossible to achieve tensile strengths >6.5 MPa.
Another way of boosting the tensile strength of silane-crosslinking systems is to incorporate short-chain diols into silane-crosslinking polyurethanes, as is described in WO 05/000931. Even with this measure, though, systems with only moderately improved tensile strengths are achievable. In applications demanding very high tensile shear strengths, therefore, it is usually not possible to employ conventional silane-crosslinking adhesives.
One of the few exceptions are silane-crosslinking systems of the kind described in WO2011/026658. In that case, very high tensile shear strengths, of up to 16 MPa, are achieved through the use of silane-terminated polyurethanes which are based on extremely short-chain polyols. One result of this is that the silane-crosslinking polymers produced have a very high density of urethane units and/or urea units that are capable of hydrogen bonding, and another is that these polymers are relatively short-chain and thus have a correspondingly high number of crosslinkable silane end groups. Inevitably however, such systems possess two inherent disadvantages. First of all, the preparation of prepolymers with a high concentration of silane-crosslinking groups necessitates correspondingly large quantities of silanes. In general, however, these silanes represent the most costly prepolymer constituents, and this raises the raw materials costs of these products accordingly. Secondly, the high concentration of urethane and/or urea groups, which is likewise necessary in order to achieve high tensile shear strengths, leads to very high prepolymer viscosities. Correspondingly problematic is the compounding of these prepolymers into fully formulated adhesives, and also the application of these end products, which usually likewise are of comparatively high viscosity.